1. Field of the Invention
This invention relates to rotary printing machines, and more particularly to web offset printing machines of either single sided or perfector (double sided) configurations.
2. Review of the Art
The maximum longitudinal length of an impression or set of impressions which can be produced by a rotary machine is determined by the circumference of the plate cylinder or cylinders, which circumference should however not too greatly exceed the length of the impression. For obvious reasons, the blanket cylinders providing the offset must either be the same diameter as the plate cylinders or an integral multiple of that diameter.
In such a machine the image to be printed is contained on the printing plate which is a thin sheet of material which is treated chemically and photographically to be ink receptive. This plate is wrapped around the plate cylinder and locked into a longitudinal slot. The blanket cylinder is wrapped with a thin piece of elastomeric material which is also locked into a longitudinal slot.
The plate and blanket cylinders are generally overpacked, i.e., the plate and blanket surface diameter exceeds the diameter of the cylinders. This sets up some interference between cylinders while establishing the pressure required to transfer the inked image from the plate cylinder to the blanket cylinder and from the blanket cylinder to the paper web.
A dynamic irregularity is created as the cylinders rotate and the longitudinal slots or gaps pass through the nip, causing printing pressure to be lost. This condition induces the cylinders to vibrate in a flexural or bending mode which interrupts the ink transfer from one cylinder to another. The result is a series of dark and light bands on the printed sheet which are referred to as "streaks".
It can be shown analytically, and is well accepted in the industry, that the "streaking" potential of a printing unit is reduced and that higher printing speeds are obtainable if the printing cylinders are arranged as nearly as possible in a common plane. Many high speed fixed circumference printing machines employ such an in-line cylinder geometry.
To provide flexible operation, it is known to provide printing machines in which at least the plate and blanket cylinders are interchangeable, typically by mounting the cylinders in an interchangeable cassette, which may also include the impression cylinder in the case of a single sided machine. A difficulty encountered with such machines is that, in view of the varying sizes of the cylinders involved, it is impossible to maintain an optimum cylinder geometry over a range of sizes of roll, and with small plate rolls, it may be necessary to resort to double size blanket cylinder. Particularly in a variable circumference perfecting machine it has been impossible to maintain in-line geometry over a wide range of plate cylinder circumferences.
Further difficulties in the design of such machines arise from the necessity of providing for cassette changing, bearing in mind that the rigidity of the machine with the cassette installed is extremely important, particularly for high speed operation. Existing machines have in general configured so that a two-dimensional movement of the cassette is to move it in or out of place, namely a first positioning movement in which it is moved in a direction parallel to the roller axes to position it alongside the machine, followed by a generally horizontal movement in a perpendicular direction to insert it into the structure of the machine and lock it in place. This requirement is wasteful of time, since precise alignment of the cassette is required in two directions. It has been proposed in U.S. Pat. No. 4,616,564 to provide a web offset printing machine of perfecting type in which an upper portion of the printer tower housing the upper inker assembly is hinged, so that it can be raised out of the way to permit a cassette to be lifted out of or lowered into position in the machine. This still requires a two-dimensional movement of the cassette, as well as the use of overhead lifting equipment. Moreover, it does nothing to overcome the other problems associated with accommodating plate cylinders of different diameters in the same machine.
A further problem that arises in machines using interchangeable cassettes is that of maintaining optimum alignment between a plate cylinder in the cassette and inking rollers in the main structure of the machine. It is known in some high quality printing machines to provide means for providing minor adjustment of the skew of the plate cylinder relative to the rubber form rollers that transfer the ink so as to maintain optimum contact between the form rollers and the plate, even when the latter is crooked or poorly installed. Loading applied to the form rollers compresses their rubber and results in a flat between the form rolls and the plate web whereby ink is transferred. For maintaining high quality printing it is critical that this flat is accurately controlled and the skewing of the plate cylinder enables this flat to be maintained at the same width across the plate. Individual adjustments may also be provided on the form rolls so as to maintain the latter accurately parallel to one another. The difficulties of providing and maintaining suitable skew adjustment of the plate cylinder relative to the form rollers are compounded in a machine utilizing interchangeable cassettes, and in consequence, such machines have not generally been regarded as being suitable for high quality printing.